TTT: non-surveyor looking for info
I expect the subject to get more discussion after everyone returns from the weekend. I’ve tried to summarize [msg=277198]one of the longest[/msg] threads we’ve had in months. If one were to print it out as it appears, it would run to near 50 pages.
If anyone, especially the original poster, can add hard facts to, or correct this summary, please do, but let’s not repeat things that have been repeated many times.
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Mr. looking4aspot, who has no surveying background, is trying to find something of significant historical and possibly monetary value buried in the ground within 25 acres in North Carolina owned by his family.He is cautious about sharing information, but once he understands what a professional surveyor can and can’t do for him, may hire one and share all the information with that person. The surveyor needs to be willing to digest all the background information and not merely stake out some numbers on the ground.
Some have suspected his post to be a troll, but others are convinced he believes what he has posted.
He believes the buried item was intended to be located from the “copious amounts of info” he has, that was prepared by “engineers, military and naval, surveyors, mathematicians, mining engineers, genious, all wrapped into a few people” around 140 years ago.”
He derives from that documentation what appears to be latitude and longitude for 9 points in an uncommon format of DD.MM.mmm North and West, that could be someone’s personal shorthand.
He appears to have no detail about how the numbers were determined 140 years ago. If his latitude and longitude numbers involved any calculation on his part, such as using bearings and distances, then he needs to tell the experts the ORIGINAL data, not something he derived from it.
He has used a consumer grade GPS (with a repeatability of perhaps 10 feet under the best conditions and often worse), aerial pictures (Google Earth?), etc, to guide the digging of large holes, and found what he thinks he was supposed to find in the holes at 8 of those points.
“The coordinates were supplied in an order…we excavated each and at each learned from our excavation where in the documents to locate the next set of coordinates, and so on.”
Some of those holes appeared to be booby-trapped just outside the target location.
He believes that digging near the 9th point without hitting the “exact” spot could destroy what he is seeking, and therefore needs the best possible estimate of that position on the ground. He mentioned geophysical measurements that turned out to not be helpful.I’m a little puzzled by the statement that the final location is “between the crossing of the north/south and east/west lines of an exact set of coordinates”. If you are just drawing lines between pairs of the 8 found monuments, then you don’t need any fancy measurements. Just go out and sight between points. If it isn’t lines between pairs of points, then what do crossings have to do with it?
He expressed the required accuracy as having the correct least significant digit in the consumer GPS display, which is 0.001 minute and corresponds to a range of about 6 feet north-south and 5 feet east-west. Undoubtedly, he would like better if possible.
It has been pointed out that 140 year old latitude and longitude would not reliably translate to any modern measurement within a hundred feet at best and probably much worse. They cannot be expected to be accurate if plugged directly into GPS, and it is astonishing if he has found 8 correct points within practical-size holes located by old latitude and longitude.
There are two principal reasons old lat-lon values won’t work: The main reason is that any measurement is relative to some framework (datum) and it may not be possible to accurately translate from one framework to another. GPS measures with respect to a global framework that has been fitted to modern studies of the earth. Old astronomical latitude and longitude measured with respect to the apparent positions of stars and/or the sun with respect to the local vertical.
The gravity vertical in various locations is inconsistent enough to affect astronomical latitude and longitude by hundreds of feet. The early government Coast Survey had its own framework (datum) that tried to account for those effects but was still many feet from today’s datum values.
Land surveys usually are with respect to local monuments on and near the subject property. Construction surveys are with respect to fixed control points on that site.
It is possible to measure extremely accurately within one framework, but translating between frameworks is difficult and rarely possible at the accuracy that can be obtained within a framework.
The second reason is that the absolute accuracy obtainable in a latitude/longitude framework 140 years ago simply was not close to modern capabilities. State boundaries marked by experts according to latitude were often found later to be 10’s to 100’s of feet from the astronomical latitude they were intended to follow, and many 100’s of feet from where a GPS latitude would fall. Astronomical longitude was even more difficult due to the need to know time very accurately.
However, the old values may accurately indicate the relation of points to each other, depending on how they were obtained. If one point and a bearing were measured astronomically, and careful angle and distance measurements on the ground of the other points were converted to calculated latitudes and longitudes, the relationships to each other could be accurate to the precision expressed, even though the location in the world was off hundreds of feet from a modern measurement.
Therefore the problem at hand is not one of measurement accuracy, but rather defining the framework within which to measure.
The best method to attempt locating his 9th point has been described by various posters in language that may not have been clear to him, and this is what he desperately needs to understand.
An expert using either professional-grade GPS gear or conventional optical instruments should measure his 8 “known” points to fraction-inch accuracy in some convenient modern framework system. A nearby 40-acre survey by the same 140-year-ago surveyors should be included if there is any data to tie it to the mystery locations.
Mathematically comparing those measurements to the old coordinates given for the found points will allow an estimate to be made of:
1) how to best translate the old framework to the modern one and therefore the best estimate of the location of the 9th point in the modern framework,and 2) how well the old coordinates after best-fit translation agree with the modern found locations as an indication of the likely accuracy of the translated 9th point.
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